1. Field of the Invention
The present invention relates to an optically dimerized high molecular compound, a liquid crystal aligning film using the same and a liquid crystal display element using the above aligning film.
2. Description of the Related Art
In recent years, displays used for word processors and personal computers are requested to be lighter and thinner and consume less power, and an excellent liquid crystal display element is expected to be developed as a flat display satisfying these requisites.
A liquid display element is provided thereon with a liquid crystal orientation film in which a suitable pre-tilt angle is set up in order to orient liquid crystal molecules to a fixed direction. Known as a method for producing these aligning films are rubbing treatment in which a high molecular compound thin film of polyimide formed on a substrate is rubbed with a cloth of rayon in one direction and a method in which silicon dioxide is obliquely deposited.
However, while the rubbing treatment is widely used in an industrial scale since it is a simple and inexpensive method, it involves such problem as production of dusts and generation of static electricity.
Further, in the method using oblique deposition, the production cost is high, and therefore it has been difficult to expand the scale to a larger one.
Accordingly, in recent years, attentions are paid to a method in which orientation treatment with light is carried out in order to solve such problems. In a process of producing an aligning film by this optical orientation method, a thin film of a photo-sensitive high molecular compound is formed on a substrate and irradiated with polarized UV-rays or laser beam, and then only photo-sensitive groups having the same directions as those of the polarized rays irradiated are subjected to photochemical reaction to cause anisotropy on the aligning film, so that the liquid crystal molecules are oriented. This method has the advantage that generation of static electricity and mixing of impurities are avoided since other members do not contact a thin film applied on a substrate.
Disclosed are several optically aligning films obtained by using photo-dimerization brought about by irradiation of polyvinylcinnamate and derivatives thereof including M. Schadt et al, Jpn. J. Appl. Phys., 31, 2155 (1992). However, these aligning films have had the defect that they are low in heat stability and shape-holding ability, so that the liquid crystal is liable to be oriented in disorder.
The present inventors disclosed in Japanese Patent Application Laid-Open No. Hei 8-254701 that an optically aligning film using polyvinyl-4-(4-fluorobenzyl)cinnamate was improved in shape-holding ability and heat stability and showed good liquid crystal orientation, but the sensitivity was not necessarily satisfactory, and irradiation with light for long time was required.
On the other hand, a polymer having an arylmaleimide residue as a photo-sensitive group having a high sensitivity on a side position is disclosed in Japanese Patent Publication No. Sho 51-13198, but application examples thereof to an aligning film for a liquid crystal display element are not found therein.